1. Field of the Invention
The present invention relates to positive electrodes for alkaline storage batteries and alkaline storage batteries using the same.
2. Related Background Art
Recently, in the field of portable information equipment such as cellular phones and lap top personal computers, there has been a strong demand for secondary batteries having a high energy density. Hence, in order to realize secondary batteries with a high capacity, a positive electrode formed by filling a porous conductive support with nickel hydroxide has been developed and is used widely. However, the simply filling of the support with nickel hydroxide cannot provide batteries with a sufficient high capacity because of a poor utilization factor of the active material. Thus, in order to increase the utilization factor of an active material, a positive electrode with divalent cobalt hydroxide powder added to nickel hydroxide powder (JP 61(1986)-49374 A) and a positive electrode with nickel hydroxide particles whose surface is coated with a cobalt hydroxide layer and a xcex2-CoOOH layer (JP 3(1991)-78965 A) have been proposed. Additionally, a positive electrode having nickel hydroxide particles coated with a cobalt compound, to which a cobalt compound further is added, also has been proposed (JP 2000-340221 A). All of these methods utilize a cobalt compound functioning as a conductant agent so as to improve the utilization factor of the active material.
According to the conventional method of adding cobalt hydroxide powder, however, a large amount of cobalt hydroxide powder has to be added in order to improve the utilization factor of the active material sufficiently. As a result, the amount of the active material (nickel hydroxide) contained in the electrode plate is decreased, which decreases the capacity of the electrode plate. In addition, as a result of the present inventors"" investigation, the coating on the surface of the nickel hydroxide particles with the cobalt hydroxide layer and the xcex2-CoOOH layer could not enhance the utilization factor of the active material sufficiently. Furthermore, the process of forming a coating layer of the cobalt compound on the surface of the active material is excessively complicated compared with the process of simply adding the cobalt compound, and therefore is inappropriate for mass production.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a positive electrode for an alkaline storage battery having a high capacity and facilitating the manufacturing thereof and an alkaline storage battery using the same.
The present inventors have found that in a positive electrode using an active material powder containing nickel hydroxide as a main component and powder made of cobalt compounds, the positive electrode with a significantly high utilization factor of the active material can be obtained by setting a ratio of a cobalt compound with a valence of cobalt being 2.6 or more to the total cobalt compounds at 30 weight % or more. The present invention is based on this findings.
A positive electrode for an alkaline storage battery according to the present invention includes first powder containing nickel hydroxide and second powder made of cobalt compounds. In this positive electrode, a ratio of a cobalt compound with a valence of cobalt being 2.6 or more to the total cobalt compounds is 30 weight % or more. Since this positive electrode eliminates the need for coating the first powder (i.e., active material powder) with a cobalt compound, the positive electrode can be produced easily. In addition, by setting the ratio of the cobalt compound with a valence of cobalt being 2.6 or more to the total cobalt compounds at 30 weight % or more, the utilization factor of the active material can be enhanced.
In the aforementioned positive electrode of the present invention, a weight ratio between the first powder and the second powder may be as follows: the first powder: the second powder=100:X, where 2xe2x89xa6Xxe2x89xa625. With this configuration, a positive electrode capable of making up a high-reliability battery can be obtained.
In the aforementioned positive electrode of the present invention, the cobalt compound with a valence of cobalt being 2.6 or more may be xcex2 cobalt oxyhydroxide.
In the aforementioned positive electrode of the present invention, the first powder may include nickel hydroxide containing at least one element selected from cobalt, zinc, cadmium, magnesium, yttrium, ytterbium and erbium in a solid solution state. With this configuration, a positive electrode capable of making up a battery with excellent properties can be obtained.
In the aforementioned positive electrode of the present invention, the cobalt compounds may contain at least one element selected from cobalt, zinc, cadmium, magnesium, yttrium, ytterbium and erbium in a solid solution state. With this configuration, a positive electrode capable of making up a battery with excellent properties can be obtained.
An alkaline storage battery according to the present invention includes a positive electrode, a negative electrode, a separator and an electrolyte, where the positive electrode is the aforementioned positive electrode for an alkaline storage battery according to the present invention.